systemd-remount-fs.service is an early boot service that applies
mount options listed in fstab(5) to the root file system, the /usr
file system, and the kernel API file systems. This is required so
that the mount options of these file systems — which are pre-mounted
by the kernel, the initial RAM disk, container environments or system
manager code — are updated to those listed in /etc/fstab. This
service ignores normal file systems and only changes the root file
system (i.e. /), /usr and the virtual kernel API file systems such
as /proc, /sys or /dev. This service executes no operation if
/etc/fstab does not exist or lists no entries for the mentioned file
systems.
For a longer discussion of kernel API file systems see API FileSystems[1].

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systemd 234 SYSTEMD-REMOUNT-FS.SERVICE(8)